CH. LVI.] THE RANGE OF HEARING 797 



from the fenestra ovalis, to the other side of which the stapes is 

 attached ; the scala tympani leads to the fenestra rotunda ; the two 

 scalse communicate at the helicotrema, and are separated from the 

 canal of the cochlea by the basilar membrane, and the membrane of 

 Eeissner. C.R is the canalis reuniens leading to the saccule. The 

 cochlea is filled with incompressible fluid in an inexpansible bony 

 case, except where the windows are closed by membranes. Hence 

 every time the membrane of the oval window is bulged in by the 

 stirrup, the membrane of the round window is simultaneously bulged 

 out to the same extent, and vice versd. These changes of pressure 

 are transmitted from one scala to the other directly through the 

 cochlear canal, setting it into vibration, and through the helicotrema. 

 The range of hearing extends over 10 or 11 octaves; the lowest 

 audible tone having about 20, the highest about 25,000, vibrations 

 per second. The range varies in different people, and diminishes 

 from childhood onwards. The upper limit of hearing may be tested 



F.Oualis 

 Stapes 



Scala Vestibuli (Perilymph) 



'Canal of-Qocniea ('Endolijmph) ^ 



Scala Tympani (Perilymph) Helicotrema 



F. Rotunda 



FIG. 497. Diagram to illustrate the use of the fenestra rotunda. 



by minute tuning-forks, metal rods, or by Galton's whistle. Many 

 animals appear to be able to detect high tones which lie beyond the 

 human limit. The lower limit may be determined by very large tuning- 

 forks, or by employing very low difference-tones. 



Difference-tones are produced when two tones of different pitch, 

 m and n, are sounded together. A tone having the pitch m minus n 

 is then heard in addition to the tones m and n : also a summation 

 tone of pitch m plus n may be heard, but with greater difficulty. 

 When m and n are nearly equal, a beating tone, instead of a difference- 

 tone, results, having a pitch somewhere intermediate between m and n. 

 If the difference between m and n is exceedingly small, this beating- 

 tone alone is heard. The frequency of the beats corresponds to the 

 difference in vibration-rates, m and n. Under certain conditions the 

 difference and summation-tones (which are collectively called combina- 

 tion-tones) exist in the air; their presence being demonstrable by 

 their reinforcement before appropriate resonators. More generally, 

 however, they appear to be produced within the ear, i.e., they have 

 merely a subjective origin. The smallest perceptible difference in 

 pitch between two successive tones is about 0'2 vibrations in the 

 middle region of the piano for trained subjects. Practice effects 

 extraordinary improvement, even among the most unmusical. 



